Phosphorus recovery from wastewater is a focus area in Denmark; the aim is to recover at least 80% of the phosphorus. In order to extract phosphorus, surplus sludge from wastewater treatment plants was acidified (pH 2-4) to increase the dissolved phosphorus concentration, which then can be precipitated and recovered. Pilot-scale acidification and dewatering tests were done using sludge from three different wastewater treatment plants: plant (1) digested primary and secondary sludge, plant (2) digested primary sludge, and plant (3) non-digested sludge. Treatment of digested sludge gave the highest phosphorus release, but the acid consumption was high due to carbon dioxide stripping. The dry matter content of the acidified dewatered sludge was high (20-40%), but the dry matter content in the filtrate increased with decreasing pH. Approximately half of the dry matter content in the filtrate could be removed by introducing an additional separation step. The optimal pH for phosphorus extraction was 3, where up to 68% of the phosphorus was dissolved. Part of the released orthophosphate was lost with the filter cake but still, 60% of the total phosphorus content in the sludge ends up in the filtrate.
Keywords: Phosphorus; acidification; dewatering; reuse; sludge.